Currently, dimensions of integrated circuits continue to be scaled down while signal frequencies continue to increase. Scaling down the dimensions of integrated circuits and higher frequencies may put limitations on use of the electrical interconnects, especially for the longer global interconnects. On-chip optical interconnects have the potential to overcome limitations of the electrical interconnects, especially limitations of the global electrical interconnects. A typical optical interconnect link includes a photodetector. Two of the critical parameters of the photodetector are the dark current and the signal-to-noise ratio (“SNR”). The dark current is generally defined as a current that flows in the photodetector when there is no optical radiation incident on the photodetector and operating voltages are applied. Generally, the signal-to-noise ratio (“SNR”) may be defined as a ratio of a photocurrent (“signal”) to a dark current (“noise”). Therefore, a low dark current is essential for the photodetectors to have a high SNR.
One of the key components of an on-chip optical interconnect link is a photodetector. In order that optical interconnects be useful with today's prevailing microelectronic processes, it is important that such a photodetector can be fabricated using silicon process-based technology and that the method of fabrication can be incorporated in a silicon process flow. A metal-germanium-metal (“MGM”) photodetector grown on a silicon substrate is one such example. Due to the presence of high density of interface energy states, the work function of a metal at a metal-germanium (“M-Ge”) interface is pinned at an energy level within approximately 50 meV of the Ge valence band edge independent of the type of contacting metal. Such pinning of the work function renders a metal germanium interface ohmic. Generally, the ohmic type contact has linear and symmetric current-voltage characteristics. The ohmic MGe interface results in a high value of the dark current. The dark current of the MGM photodetectors having such ohmic contact increases by several orders of magnitude and leads to a poor SNR, which is not desired for the performance of MGM photodetectors.
Currently, one way to reduce the dark current of the MGM photodetector involves passivating the surface of Ge by depositing an insulating silicon oxide on a surface of germanium. Another way to reduce the dark current of the Ge photodetector involves inserting an insulating amorphous Ge (“α-Ge”) layer between the metal (e.g., silver) contacts and a germanium channel layer. FIG. 1 shows a cross sectional view 100 of an MGM photodetector having an insulating amorphous germanium (“α-Ge”) layer 103 between silver contacts 104 and a germanium channel layer 102. As shown in FIG. 1, an epitaxial germanium channel layer 102 is grown on a silicon substrate 101. As shown in FIG. 1, an insulating amorphous α-Ge layer 103 is inserted between silver contacts 104 and germanium channel layer 102. Insulating amorphous α-Ge layer 103 forms an insulating tunnel barrier at the interface between germanium channel layer 102 and silver contacts 104.
Inserting an insulating layer between the metal contacts and the germanium channel layer, however, may reduce the photocurrent that flows through the metal-semiconductor interface. Reduction in the photocurrent affects the overall performance of the MGM photodetectors. Additionally, inserting the insulating layer between metal contacts and the germanium channel layer may reduce the electric field available in the germanium channel region thus reducing the photocurrent and possibly the SNR further.